The digital processing of video signals, e.g., in home video tape recorders (VTRs), allows replacement of analog video signal processing components for improvements in picture quality, performance, and ease of providing multiple or programmable functions. In conventional digital video signal processing circuitry, in the recording mode, the incoming composite video signal is converted to digital form and separated into a chrominance signal and a luminance signal. The luminance signal is frequency modulated to a standard color subcarrier frequency, while the chrominance signal is frequency-converted down to lower, color-under frequencies. The processed luminance and chrominance signals are then added together, D/A converted, amplified, and applied to recording/playback heads for recording on a video tape.
In the playback mode, the reproduced analog signals played back from the video tape are A/D converted, the frequency-modulated luminance signal is demodulated, the chrominance signal is frequency-converted back to its original state, and the two processed signals are added together to form a resulting digital composite video signal, which is then D/A converted and fed to a video signal output.
One example of a prior art digital video signal processing circuit is described in "Tagungsband, Teil 2, 12 Jarhrestagung der FKTG" (the German Television and Cinematics Society), pages 718-731, Mainz, West Germany, June 1986, compiled by G. Drechsler. As shown schematically in FIG. 2, the prior art circuit has an A/D converter aw at its input end, a first mixer q1, which is coupled to the color subcarrier frequency by a phase-locked loop comprising phase demodulator c, filter tp1, adder a1, and a first oscillator o1 outputting signals in quadrature phases (sine and cosine), an automatic gain control circuit, a chrominance signal processor cp which utilizes the chrominance signals mixed in the quadrature phases to output two color difference signals, a second mixer q2 which receives a mixing signal input in quadrature phases from a second oscillator o2, an adder a3 which adds the luminance and chrominance signals together, and an output D/A converter dw which passes the output signal to an amplifier rc and magnetic heads h.
In the recording mode (R), the input composite video signal fb is A/D converted, the luminance signal is frequency modulated through luminance processor lp, and the chrominance signal is mixed and coupled to the color subcarrier frequency through mixer q1 and the associated phase-locked loop which set the color subcarrier frequency in response to input from a mixer frequency switch unit mf. The chrominance signal is processed through chrominance processor cp and mixer q2 and associated oscillator o2 which sets the color-under frequency in response to the mixer frequency switch unit mf. In the playback mode (P), the input and output switch positions are changed so that video signals reproduced by the heads h are input to the digital signal processing circuit, and output composite video signals fb, are output from the VTR.
The above-described prior art has the problem that residual phase errors can occur in the chrominance signal processing and cause quadrature errors, which in turn become noticeable in the form of cross-color interferences between the two color difference (chrominance) signals.